The present invention relates to solubility of dopants in semiconductor materials, particularly to enhancing the solubility, and more particularly to a method for enhancing solubility of boron and indium in silicon.
Ion implantation allows for exceptional control and reproducibility in the introduction of dopants into the near-surface region of semiconductors. As a result, it has been the universal method of choice for doping MOS transistors in silicon-based integrated circuits since the beginning of the semiconductor revolution. However, an undesirable effect of ion-implantation is that it introduces significant damage into the silicon wafer in the form of point defects and their clusters, Fahey et al Rev. Mod. Phys. 61, 289 (1989). For a device to be operational these defects must be removed and the dopants electrically activated through high-temperature annealing. The annealing procedure leads to unwanted dopant diffusion, as well as nucleation and growth of dopant clusters and precipitates which results in incomplete activation.
Experience has shown that the solubility of boron in silicon under non-equilibrium thermodynamic conditions prevailing during the annealing procedure, i.e., in the presence of excess silicon self-interstitial atoms, is lower than its equilibrium thermodynamic value. The latter thus determines an upper bound for the concentration of substitutional B atoms in silicon. As technology continues to evolve toward smaller and faster transistors, this limit may soon be reached unless new ideas and/or technologies are brought forward that can reduce dopant diffusion during processing while at the same time increasing their electrical activity, see Packan, Science 285,2079 (1999).
The most widely used p-type dopant, i.e., boron, has a maximum solubility of less than 1 at. % in silicon at the annealing temperature of interest. This sets the limit for the highest concentration of electrically active boron impurities that can be reached with current implantation techniques. Already the next generation of transistors will be dangerously close to this solubility limit. Another p-type dopant candidate with excellent diffusion properties, i.e., indium, has been used only on a small scale mainly because of its very low solubility in silicon. Thus, there is a need to remedy this acute problem faced by the semiconductor industry.
The present invention provides a solution to the above-reference problem by a method for enhancing the solubility of boron and indium in silicon. The invention is based on the use of first-principles density-functional theory (DFT) in the local-density approximation (LDA) to calculate the temperature dependence of the equilibrium solubility of boron and indium in crystalline silicon under various strain conditions. Verification of this invention has shown that the equilibrium thermodynamic solubility of significantly size-mismatched dopants in silicon, such as boron or indium, can be raised by more than 100% if the silicon substrate is strained appropriately.
It is an object of the present invention to increase the solubility of dopants in semiconductor substrate materials.
A further object of the invention is to provide a method for enhancing the solubility of dopants in silicon.
Another object of the invention is to provide a method for enhancing the solubility of boron and indium in silicon. Another object of the invention is to utilize strain in a semiconductor substrate to increase solubility of dopants therein.
Another object of the invention is to induce tensile strain or compression (bi-axials) strain under elevated temperatures for increasing solubility of dopants in semiconductor materials, such as silicon.
Another object of the invention is to enhance the solubility of dopants, such as boron and indium, in silicon by at least 100% using a 1% compression or tensile strain at an elevated temperature.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention involves increase the solubility of dopants in a semiconductor material. More specifically the invention involves a method for enhancing solubility of boron and indium in silicon. As a result of the method of this invention the solubility of a dopant, such as boron or indium, in a semiconductor material, such as silicon, is increased by over 100%. This is accomplished by imposing a strain of the semiconductor material under an elevated temperature. Thus, this invention greatly advances the state of the art of silicon processing for manufacturing of integrated circuit devices, for example. It has been determined by this invention that the equilibrium thermodynamic solubility of size-mismatched impurities, such as boron and indium in silicon, can be raised significantly if the silicon substrate is strained appropriately. For example, a 1% compression (bi-axial) strain of silicon raises the equilibrium solubility of boron in silicon by 100% at 1100xc2x0 C., or 1% tensile strain of silicon at 1100xc2x0 C. enhances the solubility of indium in silicon by 200%.